With a known bone plate of this kind (EP 0760632 B1), the holes are all around spherically formed towards the upper side of the bone plate, in order to be able to support a bone screw with a head having a spherical lower side, in different angular positions. At the plate lower side, adjacent to the bone, the holes have a region of smaller diameter with partial thread in order to be able also to accept a bone screw with cylindrical threaded head which screw shall be inserted, vertically to the plate plane.
With a further bone plate of the specified kind (EP 1158915 B1, and EP 1198916 B1), elongated holes with an internal thread are available which thread extends from the upper side to the lower side of the bone plate, at one end of the elongated hole, and has a peripheral or central angle in the range from 190 to 280°. The internal thread extends over the whole depth of the elongated hole, tapers towards the lower side of the bone plate, and has a cone angle in the range from 5 to 20°.
With a further known bone plate (EP 1255498 B1), elongated holes are provided in the bone plate which holes may be formed oval, ellipsoidal, or rectangular, or may have a combination of such forms. Only circular holes are explicitly not covered by this definition of an elongated hole. The elongated hole is combined with a circular hole, and the latter is provided with a three-dimensional structuring which is available in the form of an internal thread, or a peripheral plate or lip. A conical internal thread is shown which extends from the upper side to the lower side of the bone plate, and has a peripheral or central angle in the range from 190 to 280°.
A fixation system for bones, having a bone plate is known from DE 19858889 A1 which system has elongated holes which have lugs closely to their lower sides adjacent to the bone. The lugs extend in the lower part of the elongated hole, in parallel to the plate plane. There are bearing surfaces for spherical heads of the bone screws, towards the upper side of the bone plate. For cooperating with the lugs of the bone plates, the bone screw has a short piece of thread, below the spherical head which thread is able to deform the lugs in the elongated hole, and to adapt. It is thereby possible to screw the bone screw in different angle positions with respect to the axis of the passage holes.